1. Field of the Invention
This invention relates to controlled release nitrogen fertilizers and methods for their preparation. More particularly, the present invention is directed to a method of forming a new highly available slow release nitrogen fertilizer by condensing ammonia, urea, and formaldehyde under mildly basic conditions and elevated temperatures and then dehydrating the condensate without substantial polymerization to a controlled release solid fertilizer by means of a weak acid catalyst at elevated temperatures and short reaction times. The method provides granulation of homogeneous solids in a high intensity reactor either as a controlled release nitrogen, or complete N-P-K fertilizer. The new composition provided by this method provides higher nitrogen availabilities in high controlled release nitrogen concentrations than heretofore available.
2. Description of Related Art
Ureaformaldehyde condensation and polymer products are widely used as slow release nitrogen fertilizers for lawns, ornamental plants and crops. These products usually contain methylene urea polymers having low water solubilities and release nitrogen over an extended period of time. The methylene urea polymers decompose by microbial or enzymatic action to water soluble mineral nitrogen.
Prior art workers, such as Goertz in U.S. Pat. No. 4,378,238, disclosed that cold water soluble ureaformaldehyde products consisting of methylene diurea and dimethylene triurea were effective, non burning, particulate fertilizers.
In U.S. Pat. No. 5,102,440 Gallant et al minimized the formation of HWIN by using very high urea to formaldehyde reaction mol ratios and by spraying a liquid urea formaldehyde resin on cool fertilizer raw materials; thereby, forming granules containing high concentrations of free urea.
In the prior art methods, slow release fertilizers are usually prepared by reacting urea and formaldehyde in aqueous alkali solutions to form methylolurea, and then acidifying to form methylene urea polymers. Some workers, such as Greidinger et al in U.S. Pat. No. 4,089,899, maintain low hot water insoluble nitrogen (HWIN) and high free urea contents by carrying out the reaction at low temperatures with large excesses of urea in the reaction mixture. Although the low HWIN is desirable, the high free urea is not because it increases phytotoxicity of the fertilizer significantly, and causes severe granulation difficulties by creating sticky operating conditions. A high process recycle ratio and low process temperature must be used to granulate fertilizers containing high free urea contents because very small amounts of water cause urea containing solids to become sticky and difficult to granulate. When the materials produced at low temperatures are dried at normal drying temperatures, the HWIN usually increases substantially unless some special means of drying is used.
In my U.S. Pat. No. 5,266,097, the introduction of ammonium ion, for example as ammonium sulfate, along with urea and formaldehyde in an acid catalyzed polymerization is used to produce controlled release fertilizer solids. The amount of CWIN was limited by the necessity of including at least 0.05 mol of ammonium per mol of formaldehyde. The amount of ammonia required and the anion required to neutralize it, undesirably increased the osmotic pressure, or osmolality, of the fertilizer. Increasing osmolality has been shown to increase the phytotoxicity of a fertilizer.
Hawkins, in U.S. Pat. No. 4,554,005, reported a heterocyclic aminoureaformaldehyde, s-triazone, to be an effective controlled release liquid fertilizer.
My U.S. Pat. No. 5,449,394 disclosed a heterocyclic, non-polymeric condensed controlled release liquid fertilizer containing substantially 5-methyleneureido-2-oxohexahydro-s-triazine, prepared by the reaction of formaldehyde, urea, and ammonia under near neutral reaction conditions at about 100.degree. C.
Ureaformaldehyde fertilizers have been evaluated in the past by the amount of cold water insoluble nitrogen (CWIN) contained and by the release characteristics of the CWIN. When the CWIN is not soluble in hot water, it is known that the nitrogen is unavailable for plant utilization in the soil for a very long time, if ever. This hot water insoluble nitrogen is undesirable, although many fertilizers found in commercial use have 60 percent or more of their CWIN in the form of hot water insoluble nitrogen (HWIN).
Experience in recent years has shown that the water soluble nitrogen products of the aforementioned U.S. Pat. Nos. 4,554,005 and 5,449,394 are indeed controlled release and exhibit lower phytotoxicity and lower, longer availability than urea nitrogen. A realistic evaluation of the controlled release efficiency of a fertilizer may be made using the following equation: ##EQU1## Terms used herein are defined as follows: Urea-N=urea nitrogen, not controlled release;
NH.sub.3 -N=ammonia nitrogen, not controlled release; PA1 CWSCN=cold water soluble condensed nitrogen, controlled release; PA1 CWIN=cold water insoluble condensed nitrogen, controlled release; PA1 HWIN=hot water insoluble polycondensed nitrogen, controlled release; PA1 CRE=controlled release nitrogen efficiency factor; and PA1 Hydroxymethylnitrogen compounds=simple condensation products from the reaction of formaldehyde with the nitrogen compounds urea and/or ammonia, retaining the hydroxymethyl group and substantially free of the methylene group; PA1 Dehydrating reactor=a reactor wherein the hydroxymethyl nitrogen compounds are catalytically reacted to split water from the hydroxy-methyl compounds to convert them to methylene compounds; PA1 Methylene nitrogen compounds=nitrogen compounds in which methylene groups connect nitrogen groups into controlled releasing nitrogen compounds, in which ammonia or urea comprise the nitrogen groups. The nitrogen may be in either linear or cyclic compounds.
Controlled Release Nitrogen=CWSCN+CWIN Single growing season availability=CRE=portion of controlled nitrogen available to plants in a single growing season;
A simple condensation reaction may occur when compounds such as urea, or ammonia, react with formaldehyde to form a single compound such as methylolurea, or methylolamine. A simple dehydration condensation reaction may occur when two simple compounds, such as methylolurea combine by splitting out water to form a single compound, such as methylene diurea. A polycondensation dehydration reaction may occur when a plurality of compounds, say six for example, such as methylolurea condense to form a polymer, such as hexamethyleneheptaurea.
The prior art has been based on allowing the formaldehyde, urea, and ammonia, reactions to proceed to equilibrium. Therefore, whenever low urea to formaldehyde ratios, relatively high dehydration reaction temperatures, and effective dehydration catalysts have been employed, polycondensation dehydration reactions occurred. These cause the formation of undesirably large amounts of hot water insoluble nitrogen, which is unavailable to plants in a single growing season. The polycondensation also results in the production of undesirable free urea from the intermediate methylolureas even when relatively low urea to formaldehyde mol ratios are used.
There have been no prior art teachings in which the urea, formaldehyde, ammonia system reaction was used to obtain high simple condensation conversion at a high rate, and the condensation reaction stopped before equilibrium was reached where polymeric condensates and free urea were produced. Instead, the prior art has used techniques such as adding excess urea to retard the polycondensation, or used weak reaction conditions such as low reaction temperatures and weak dehydration catalysts to slow the reaction.
The prior art has, to a large extent, been directed to increasing the portion of slow release condensed and low polymeric ureaformaldehyde nitrogen and decreasing the amounts of higher polymers in the fertilizers. Unfortunately, these prior art improvements have been achieved by increasing other undesirable properties of the controlled release nitrogen fertilizers, such as increased free urea concentrations and burn potentials.